Cathode of electron beam devices



Jan. I, 1952 w. J. moons CATHODE OF ELECTRON BEAM DEVICES 3 Sheets-Sheet 1 Filed May 28, 1949 Jan. 1, 1952 Filed May 28, 1949 w. J. moons CATHODE OF ELECTRON BEAM DEVJ ICES 3 Sheets-Sheet 2 I N r- 4 q) q Egg i". I hi I i f q) 1 E l LJJ.

INVETOR Wellgsleyi d3 Jan. 1, 1952 w. J. 00005 CATHODE OF ELECTRON BEAM DEVICES 3 Sheets-Sheet 3 Filed May 28, 1949 Welleslqgj $5 Patented Jan. 1, 1952 UNITED STATES TENT GFFHC Wellesley J. Dodds, Cranbury,

Radio Corporation of Delaware N J., assignor to irAmerica,- a corporation Application May 28, 1949, Serial No. 96,068 7 Claims. (01. 313338) This invention relates. to improvements in the cathodes of electron beam devices. More particuiarly itrelates to improvements in electron-gun cathodes whereby certain conditions'under which they operate, and thereby certain of their electrical characteristics, may be adjusted to control the forming of the beams of electrons.

In prior-art-arrangements of the electrostatic lens systems of electron-guns, cathodes-have provided the electrons for the beams -(by their emissivity) and theyhave played a part in controlling the. beam thickness (by the size of their emissive areas) However, they have not otherwise played an important part in controlling thenshape. of the beam. More specifically, while it has been the customary practice to design a cathode with suitable properties for its emitting surface to provide the required beam current and of suitable size to have a controlling effect on the size of the beam cross-section, it has not been the practice to design into the cathode structural characteristics which permit external adjustment of certain of its operating conditions to control the relative directions of travel of the electrons and thereby to influence" the shape of the beam.

Due to :the fact that in certain electron guns it has not been possible thus to control the relative directions of travel of the electrons and since space charge eiiects may result in spreading of the beam, it has been necessary to use very strong axial magnetic fields to guide them. This has had the disadvantages of making the equipment bulky and expensive.

It is an object of the present invention to devise improvements for an electron-gun cathode, to permit the adjustment of certain conditions under which it operates and thereby of certain of its electrical characteristics to influence the beam-forming operation of the electron gun.

It is a further object of the present invention to devise improvements for an electron-gun cathode as setfcrth above'in which the cathode has a largeemitting area, the gun is of a type for producing a thick beam of electrons, and the beam-forming operation can be so influenced, by said adjustment, that the electrons increasingly tendto move in directions which areparallel to one another and to the gun axis.

It is a further object of thepresent invention to devise a novel electron gun having acathode which is improved as set forth above and in which the beannforming operation is influenced by. causing? d-ifierent portions of: the: large emitting surface of the cathodeto lbexat difierent static potentials to the end that an electrostatic field will be-established which will oppose undesired electron space. charge effects on the equipotential surfaces of an electrostatic lens system of the gun.

It is a further object of the present invention to devise a=novel electron gun as set forth above in which the cathode is. adapted'to have different portions. Qfitsemitting surface polarized at different static potentials tothe end that an electrostatic field will be established which will oppose-undesired electron space charge effects on the equipotential-surfaces of an electrostatic lens system of thegun-and in addition will reduce said undesired eifectsby causing the portions of the beam current which come from said diiierent portions of the emitting surface to have different predetermined magnitudes for determini-rig the valuesof electron space potentials at diii'erent points within the beam.

It is a furtherobject of the present invention to devise anovel electrongun as set forth above whereby the need for using axial magnetic fields for guiding the electrons will be reduced.

-Other-objects; features, and advantages of this invention will. be apparent to those skilled in the art from the following detailed description of +an illustrative embodiment and. from the drawing in which:

Fig. 1 exemplifies a prior-art electron gun of a type which is-intended toproduce a thick beam of electrons having a constant circular crosssection, i. e., a beam all of theelectrons of which move parallel. to the gun axis within a'space having cylindrical boundaries adjacent to a space-charge-free region;

Fig. 2 .showshow, due to space charge effects, the electron gun of Fig. 1 usually fails to perform in the intended manner;

Figs. 3; 3a, 3b:and 4 illustrate how the shortcomings of the electron gun of Fig. 1 can be eliminated by improvements made in its cathode according to the present invention; and

Fig. 5 represents an electron discharge device hayingan electrongun which includes an improved cathcdeof the type disclosed herein.

' In general,-.-according to the present invention, the cathode is so constructed that different portions" of its emissive surface can be polarized at diiierent static potentials to permit control, both directly and indirectly of' the shapes of the equipotential surfaces-produced by an electrostatic lens system of the gun. It permits their control directly in that the absolute space potentials which exist in the lens system and produce the equipotential surfaces are determined in part by the values of these different static potentials which are applied to different portions of the emissive surface, and it permits their control indirectly in that, as will be explained more fully below, the densities of the electron space charge at different points within the beam, and therefore the effects of the electron space charge as a whole on the absolute space potentials, are also determined in part by these values.

In Fig. 1 there is shown an electron gun I comprising a grounded thimbleshaped cathode 3 having an emissive coating 5 and a heater 1. The coating 5, in the example shown herein, is carried on a flat circular surface of the cathode 3. A zero-potential electrode 9 includes a dishshaped rim l and a central cylindrical extension ll within which the thimble-shaped cathode 3 is supported so that the coating is coextensive with the concave surface of the dish-shaped rim 10, i. e., with its surface which faces in the direction of electron travel. Positioned along the gun axis are three dish-shaped accelerating-andfocusing electrodes l3, I5 and 11 each having a central opening of the same size and shape as the emissive coating 5.

It is intended that the electrostatic lens system comprising the electrodes l3, l5 and I! should cause the electrons emitted from the cathode 3 to move in directions parallel tothe gun axis to form a cylindrical beam having a constant circular cross-section equal to the emissive coating 5. To attain such performance it is essential that the electrodes l3, l5 and I! be appropriately formed and polarized so that, as in the examples represented at 2|, 23, and 25 in Fig. 1, each equipotential surface which is established within the lens system will be fiat in its portion which lies athwart the intended beam and will be perpendicular to the direction of beam travel. Principles by which one may be guided in determining the proper surface configurations and polarizing potentials for obtaining such equipotential surfaces are known in the art, for example, ones which are particularly appropriate for designing the type of lens system shown in Fig. 2 are set forth in the article Rectilinear Electron Flow In Beams by J. R. Pierce which was published in August 1940 in volume II No. 8 of the Journal of Applied Physics.

However, considerable difliculty has been encountered in making thick beam focusing systems according to these known principles. For one thing, most of the formulae to which these principles have been reduced have been derived from analyses which have not fully accounted for space charge efiects; in addition, though each electrode of the lens system is a rigid structural element out of reach within a glass envelope (so that it is impossible physically to change its surface configuration either to correct imperfections therein or to compensate for changed operating conditions) no means has been available for electrically compensating for unsatisfactory surf ace configurations;

and finally, even if a focusing system could be made to operate properly for a specific set of operating conditions it would not operate properly for certain different operating conditions.

This is apparent in view of the fact that, for example, the extent to which the electron space charge afiects the shape of .the equipotential surfaces will change with variations in either the current density of the beam or its velocity.

Fig. 2 illustrates how the electron space charge affects the shape of the equipotential surfaces in their critical regions i. e., their regions through which the beam must pass. As is known, the electrons of a beam which is of uniform density and is surrounded by a space-charge-free region do not produce the same negative space potential at different points across the beam. Instead they produce the strongest negative space charge near the center of the beam and therefore it is in this region that the positive absolute s ace potential afforded by the electrostatic lens is most depressed. In portions of the beam nearer to its boundaries the space charge also tends to have depressing eifects but to lesser degrees. As a result of this, as shown in Fig. 2, an actual equipotential surface 2!, which is produced intermediate the electrodes 9 and I3, has its central portion depressed away from the cathode during operation of the gun, i. e., during the emission of electrons, so that it ceases to lie in a fiat plane. Correspondingly, other actual equipotential surfaces, such as the equipotential surfaces 23 and 25 which are respectively produced between the pairs of electrodes i3, [5, and IS, IT, have their central portions depressed away from the cathode. For known reasons, this has the effect of causing the electrons of the beam to diverge rather than to travel in parallel lines.

To. obtain the desired operation despite these shortcomings of electrostatic lens systems it has been customary to use strong axial magnetic fields. As is known this results in beams of parallel-moving electrons due to the fact that a magnetic field acts to confine each electron to a path of travel close to the single line of magnetic force nearest to its point of emission from the cathode, i. e., close to a line which is parallel to the gun axis. However, the electromagnets needed for this purpose are very large and therefore they the expensive both to build and to operate.

I have considered as a possible solution the forming of the end surface of the cathode 3 (its surface which carries the emissive coating 5) to be concave in the direction of electron travel. Such a shape would increase the distance between the center of the coating and the center of each equipotential surface ahead of it thus lowering the gradient of the accelerating potential acting on electrons hovering in the space charge in front of the center of the emissive coating and reducing the density of the space current drawn from it. This would result in a reduced space charge at the center of the beam and therefore in a lessening of the depressions in the configurations of the equipotential surfaces.

However, while this does tend to help, it is not a complete solution; it lessens space charge effects but does not eliminate them; the curvature of the concavity, once selected, would be unadjustable and would have to remain fixed; and, as is known, concavity of a large area cathode surface which extends crosswise to a gun axis tends to produce undesired eifects due to the electrons having initially convergent directions of travel as they are emitted. Accordingly, I have devised the presently-disclosed improvements in the construction of the cathode which make it possible both to lessen and to oppose space charge efiects on the electrostatic lens and to do so in a controllable manner. The improvements may be employed with either a flat or concave emitting surface for the cathode.

More particularly the improved structure makes it possible for different portions of the emissive area of the cathode to be polarized at difierent potentials. As :shown in ith'e'irFigflr 3 embodiment of the" inventionthe cathode..-may1be composed of a plurality of component parts electrically-insulatedirom each' other so that lt' -rn'ay be connected to an external-circuit for polarizing the difierent component parts at different and individually variable potentials. 'Inorder'to s mplify Fig. 3 the'cathode is shown to comprise only four-such component parts. However, in practice it maybe desirableto use a larger number'and accordingly it is not intended, by the example which is-shown in the drawing; to indicate any necessity for limiting the extent to "whichlthe cathode :istsubdivided.

At'ithe center of the cathode there 'isa'thimble shaped component 3a closedat its front end to provide a smallcircular area for 'ca'rryingfa'n emissive coating 4a. As is. more readilytapparent from Fig. 3a the:di ameter of component 3a is aboutnne's'eventh. of that ofthe entire cathode 3'." Surrounding component- 3a is acoinponent" 3b formcdas a sleeve which is re-entrant'upon itself, i. e.,iformed asltwo'ponc'entric sleeves joinedat the "emitting" end of the'catho'de by 'a' narrow annu ar ring whichcarries an emissive coating db (this coating affords the next to-the -"center subdivision of the total emissive area of the cathode). The inner sleeve' of component 32) surrounds the 'thirnble-sh'apedcomponent 3a and they are mechanically joined together by an intermediate :layer Bof an insulating materialxsuch as aheat-resistant ceramic. "Componenteb is surrounded 'by a component 30 which is. shaped like it but i large enough so thatrlts' inner sleeve surrounds the outer sleeve of component" 3b.- Components 3b and-3c areralsomechanically joinedtogether-by an intermediate layerof insulating material. Similarly,"-an' outer' component-ad-surrounds and is joined to'thc component 30. From-the foregoing it is apparentthat While the cathode: is a unitary structure structurally, electrically it"is-mad'e 'up of isolated components and that each'ofzthese" components ."provides a subdivision of the 'total emissive area ofv the cathode which is. symmetricaround'the center thereof. The" components 3a+3d mayrbe heated in any suitable-manner, such as :by separate resistance windings (not shown) which areinserted in their respective open ends and supported within them by ansinsulating ceramic material which is capable of withstanding'the ;heat required to operate the cathode.

. In Fig. 311211001; Sitrepresents a'source of-adjustable polarizing potentials for the individual components 3a 3dfof the: cathode 3. purposes otillustrationthe source of polarizing potentials isshown to comprise a .batteryss which is grounded :atritsinegative end and is shunted by a voltageedivide'r 35 having three adjustable output taps which are connected individually to the com'-' ponents So, 322 and 3c." Theoutermost-component of the cathode-,- component 3d, is grounded; lhe source S-i-should'be" capable of applying to the components 312-30 potentials of sufiiciently greatmagnitudes to draw the center portions'of the equipotential surfaces toward the cathode to produce distortion opposite. to andsubstantially equal to thoselillustrated in .Fig. 2) which are produced by the electron space charge. Since the negative space charge :will itself bereduced, due to the reduction in space currentwhich this type of-cathode construction will-cause in the center. of the beam: it willc-be:.'found;that. th'e amount "of depression .produced by the space For the:

charge on: the 1 positive :equipotential surfaces iwill itself besomewhat lessened.

In: order to obtain :the bestiperiormance for an electron gun LbuiIt accordingv'to EthiSliIlVBDfiOlT it maybe necessary-to try: diiferent combinations of positions-for ithe taps? of. the source of=:potentials: 3 I 1 continuin'gito do thislduring actualroperation until-parallel ffl'ow of the beamelectrons is ':obtained. "Ther a ;'.;number:.of i'known testsby whichthis condition ma'yrbe recognized. For-:example, if zero potential were-initially applied: to all of the cathode componentssoflthat the electrons: move inidivergent directions; the-proper adjustments would bexprogressively.to' increase "the magnitude 6 of the :positive potential applied to the. center: component of the cathode and .for each increase t'o "try: different combinations of positive potentials rorsi-ts other. components (except for the outer Pcomponent; which' will usually, the not :necessarily always; be "at-zero: potential) untilv minimum return 'cu-rrentsare' observed to flow between; 'the'cath ode 3 and. the: individual electrodes I3, lfiandll. .As already indicated. above, this will lead to a diminuti'omof thespace charge effects. as i such so that rigorously considered the: potentialsrapplied t'o'ithe components of ltheicath'ode' willinot: have to produce opposing distortions. (of. thec'equipotentialsurfaces) which are fully. as large as those which would-be produced; by::'spacei charge 5 effects if a 'unipotential cathode .were. bei11g used;'as'.:in Fig. '2.

I An alternatezform of: cathode: construction for practicingthe: present invention is shown in Fig. 3b. In itfthe'cathode consists of adirectly'heat'ed filament -1 whichcsis iwound in spiral form. J In operation of the gun t 'efinner end '37 of the filament fSG. is connected: to theipOSltiVe side o'f'a source: of directifpotentialiss :and-Jitsiouter' end '39 is-zconnected to :itscnegative. side which: may be grounded Iir-this away the S0ll1Cef38i1'Sel Ves both to heat the filament and? to establish'Talong 'its length a voltagesgradientfor.polarizing the center of the cathbdef'at a; positive value which progressively I decreases in rm'oving outwardly to its perimeter. If it is desired. :to establish a gradient of-X- volts 'between the outer'fedge of the cathode and its cententhen the sourceztfi Eshouldprovide a direct potentialmf r X volts and :the resistan'ce valuei-of the filament sit-should :be such. that it willwbe appropriately heated :for' proper emission when it:'i oconnectedvacross"this D. C. source. The spiral-- may be .Wound :with progressively large or" smallervspacings between' successive pairs of adjacent turns '50 a'ssto control the rate: at which thegradient*varies-between the center of the cathode and its edge.

. For reasons-'rwhichware'zobvious in :view of the foregoing, for some embodiments it:may"be-advantageousto-uconnect the negative side of the sourcersa to ground through a biasing source of adjustable-value.

- Figure rlfrshowsthowlimproved performance is obtainedzfor"ancelectron vgunof the type shown in'iFigure 1 by modifying its-cath ode-according to theapresentinvention. -In it line 4| repre sents an equipotential surface which'would' be producedvin this 'electron gundue to the proper design of its electrodes 9, 13,15, and I? even Without-the application of m'ultipotential polarizationto its cathode provided the the cathode is cold, i. e .;-prov-ided there is no emission. Due to the absence of space charge this" properly designed lens system would produce equipotential surfaces having 7 portions which are flat and perpendicular to the gun axis where-they=cross the intended path of the beam. Thus surface 4| corresponds to the surface 2| of Fig. 2 and has the configuration needed for best operation of the gun. Line 43 represents the same equipotential surface as distorted only by space charge effects. In other words, line 43 represents the equipotential surface which is produced at the source position between the electrodes 9 and I3 when the situation is entirely the same except that the cathode is heated to obtain electron emission. Thus, line 43 corresponds to line 2| of Figure 2, which, of course is to be expected, since all of the components of the cathode are assumed to be at ground potential, i. e., since, in effect, the improved cathode is no different from the unipotential cathode shown in Figures 1 and. 2. Line 45 represents the equipotential surface 4|, as distorted only by the application, to the different components of cathode 3, of appropriate polarizing potentials for influencing its shape by the same amount as, but in the opposite direction from that by which it is influenced by space charge effects when the cathode is heated.

It is apparent from the foregoing that under all the conditions for normal operation of the electron gun shown in Figure l, that is to say, when its cathode is appropriately heated to produce emission and the components of the oathode are appropriately polarized from the source 3|, the opposite distortions of the equipotential surface 4| produced by the electron space charge and the multi-potential polarization of the cathode will exactly cancel themselves out so that the actual equipotential surface will be exactly like the intended one shown at M. This would result in the condition shown in Fig. 1 and therefore in rectilinear movement of the electrons emitted from the cathode 3 and in the formation of a beam of the desired type.

Fig. 5 illustrates how an electron gun including a cathode which is improved as disclosed herein may be used in an electron beam device of the type now known as a traveling wave tube. The improved electron gun, which is represented at 5| in this figure, is enclosed in an evacuated envelope 53 and is positioned to direct a beam of electrons through a helix 55 to a collector 51. As is known a discharge device of this type may be operated as an amplifier by feeding radio frequency energy onto the helix 55 at its end nearest to the gun and withdrawing the same from the other end at which point its magnitude will have been increased by energy derived from the electron beam. As is also kown this type of device will operate most efiiciently if the electrons comprising the beam travel in parallel directions.

The present invention is not necessarily limited to producing beams having cylindrical boundaries. On the contrary, the present invention may be practiced to produce beams whose electrons move in parallel directions but in which the cross-section of the beam has some shape other than circular, such as square, or rectangular.

Likewise the present invention is not limited to guns for producing the parallel movement of electrons which form a beam. It is obvious that the multi-potential applied to the cathode may be adjusted so that in cooperation with the electrostatic fields set up by other electrodes of the lens system, the electrons of the beam may be made to diverge or converge at predetermined angles.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

I claim:

1. An electron gun for producing a beam along an axis comprising a cathode having different emissive portions which are arranged in a pattern of substantially circular symmetry about said axis and all of which coincide with a predetermined transverse plane to constitute together a substantially circular emissive surface thereat, one of said portions being positioned near the center of said emissive surface and at least one other thereof extending for a full 360 around it, said cathode being positioned with said center located on said axis, means surrounding said axis for accelerating electrons therealong and focusing them into a beam said means including an electrode next adjacent said cathode and displaced axially therefrom which has a single circular opening surrounding the intended path of the beam and concentric with said axis, means for connecting a source of potential between the cathode and said electrode for establishing an electro-static field to cause and direct movement of electrons away from the cathode and along said path, and means connected to the cathode and connectable to at least one source of direct potential for providing different polarizing potentials for the different cathode portions to symmetrically infiuence the configurations of equipotential surfaces between said cathode and said electrode.

2. An electron gun comprising a beam accelerating-and-focusing system including at least one electrode surrounding an intended path of travel for beam electrons, means for connecting a source of potential to said electrode for establishing an electrostatic field to cause and direct movement of electrons in the system, a cathode comprising a plurality of coaxial components each of which has an emissive surface, the components being supported with their emissive surfaces lying in a common surface having a predetermined over-all configuration, the components being electrically insulated from one another to adapt them for independent connection to external circuits, and means for applying individual direct potentials to said components for controllably influencing said direction of the movement of electrons by said electrostatic field.

3. An electron gun comprising a beam accelerating-and-focusing system including at least one electrode positioned along an intended path of travel for beam electrons, means for connecting a source of potential to said electrode for establishing an electrostatic field to cause and direct movement of electrons in the system, a cathode comprising a plurality of components each of which has an emissive surface, the components being supported with their individual emissive surfaces lying in a common surface having a predetermined over-all configuration, the compon-- ents being electrically insulated from one another to adapt them for independent connection to external circuits, means for applying individual direct potentials to said components for controllably influencing said direction of the movement of electrons by said electrostatic field, one of said components of the cathode having its emissive surface at the center of the emissive area of the cathode taken as a whole and each of the other components having its emissive surface positioned symmetrically around the center of said first mentioned emissive surface.

4. A thermionic cathode electrode having coaxial portions provided with separate and discrete emitting areas lying in a common surface and adapted to be maintained at different potentials with respect to each other, at least one of said portions extending for a full 360 around another thereof whereby substantially symmetrical equipotential surfaces will be established near said common surface when portions of the cathode are maintained at different potentials.

5. An electron gun assembly for a beam type electron discharge device including a. thermionic cathode structure having discrete emitting portions arranged radially outward from an axis in spaced relationship from each other and an electrode having an opening surrounding said axis and supported adjacent the outermost of said discrete portions.

6. An electron gun assembly for a beam type electron discharge device including a, thermionic cathode structure having discrete emitting portions arranged radially outwardly from an axis in spaced relationship from each other, said portions being insulatingly supported with respect to each other whereby different potentials may be 10 applied respectively thereto to produce an electric field varying in intensity radially outward from said axis, and an electrode having an opening surrounding said axis and supported adjacent to the emitting portions positioned furthest radially outward from said axis.

7. A thermionic cathode electrode having coaxial portions provided with separate and discrete emitting areas lying in a common surface, the innermost coaxial portion having an emitting area with a circular perimeter, another co axial portion having a ring-shaped emitting area concentric with and surrounding the innermost portion, and each of the coaxial portions being insul-atingly supported with respect to the others.

WELLESLEY J. DODDS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,035,623 Sukumlyn Mar. 31, 1936 2,197,033 Diels Apr. 16, 1940 2,409,514 Pratt Oct. 15, 1946 2,466,064 Wathen et al. Apr. 5, 1949 

